This invention relates to an imaging method and apparatus using nuclear magnetic resonance (NMR) techniques, which provide a clear image from a distribution of relaxation time T.sub.1 on a desired slice of a patient.
The NMR imaging method is an attractive medical diagnostic technique, having many superior characteristics including noninvasive measurement and the ability to obtain diagnostic information at the cell level. NMR signals, however, are extremely weak and have a low signal-to-noise ratio when compared with signals detected in other diagnostic apparatus, e.g., X-rays or ultrasonic waves.
In a conventional NMR imaging method, atomic nuclei of hydrogen are selected as the measured element and an image of a distribution of hydrogen atomic nuclei is reconstructed using techniques similar to an X-ray computerized tomography scanner. Since NMR signals include information on the density of hydrogen atomic nuclei and other physical parameters, in particular relaxation times T.sub.1 and T.sub.2, the image produced by NMR is greatly influenced by the combination of these parameters.
Since the experiment to measure relaxation time T.sub.1 by R. V. Damadian (See, e.g., U.S. Pat. No. 3,789,832), there have been many publications providing information on using NMR for medical diagnoses. Many researchers have reported that the magnitude of relaxation time, especially T.sub.1 (spin-lattice), is remarkably effective in diagnosing malignancy tumors or cancerous cells. If an image (hereinafter called a T.sub.1 image) can be obtained which depends only on relaxation time T.sub.1 by removing the influence of any other physical parameters, then that T.sub.1 image will assist greatly in medical diagnoses.
At present, NMR signals are usually detected utilizing the following pulse sequences:
(a) saturation-recovery method (SRM); and
(b) inversion-recovery method (IRM). These are predetermined pulse sequences including selective excitation pulses (SEP or 90.degree. pulses) and 180.degree. pulses which will be described later.
The image derived from the SRM pulse sequence is hereinafter called an SRM image and the image derived from the IRM pulse sequence is hereinafter called an IRM image.
The signal intensity I of each pixel of these images is expressed approximately as follows: EQU SRM: I.alpha..rho.(1-e.sup.-T.sbsp.1.sup./t.sbsp.1)e.sup.-T.sbsp.2.sup./t.sbsp. 2 ( 1) EQU IRM: I.alpha..rho.(1-2e.sup.-t.sbsp.0.sup./T.sbsp.1)e.sup.-t.sbsp.3.sup./T.sbsp .2 ( 2)
where, .rho. is a density of hydrogen atomic nuclei, T.sub.1 is a spin-lattice relaxation time, and T.sub.2 is a spin-spin relaxation time. In the SRM pulse sequence, t.sub.1 and t.sub.2 indicate the repetition time intervals for the selective excitation pulse and spin-echo interval, respectively. In IRM pulse sequence, t.sub.0 and t.sub.3 indicate a waiting time between the first 180.degree. pulse and the selective excitation pulse and spin-echo interval, respectively. The spin-echo interval is a period from selective excitation pulse to echo-signal to be measured.
In a conventional diagnostic apparatus utilizing NMR techniques, an SRM image or IRM image is reconstructed from echo-signals and displayed directly for diagnostic purposes. Generally, there is a close correlation between relaxation time T.sub.1 and relaxation time T.sub.2, since tissue or cells having a large relaxation time T.sub.1 usually have a large relaxation time T.sub.2. Consequently, each magnitude of the exponential of the relaxation times T.sub.1, T.sub.2 in equations (1), (2) cancel each other. The SRM image or the IRM image, therefore, contains only weak information about T.sub.1 and T.sub.2 relaxation time.
Comparing the SRM image and IRM image, the IRM image contributes more to relaxation time T.sub.1 than the SRM image. Each signal intensity of the IRM image, as is apparent from equation (2), changes from positive region to negative region, so that the IRM image (.rho.=0, i.e., where there is no object) becomes weak. In order to overcome these drawbacks, an improved method in which a plurality of images are generated by changing measuring parameters and relaxation times T.sub.1, T.sub.2 are calculated by comparing the intensities of images, is proposed. However, these images also have drawbacks in that the signal-to-noise ratio degrades. Therefore, it is difficult to obtain the T.sub.1 image which depends only on relaxation time T.sub.1.